Method and apparatus to detect a gas by measuring ozone depletion

ABSTRACT

The present invention relates to an apparatus and method for determining the concentration of nitric oxide (NO) in a gas mixture such as air. The gas sample containing NO is mixed with a gas containing ozone (O 3 ), and the change in the ozone concentration is measured after a sufficient time is allowed for the reaction between NO and O 3  to take place and destroy a measurable quantity of O 3 . In the preferred embodiment, the concentration of ozone is measured using the technique of UV absorption. In this case, the invention has the advantage over other instruments for measuring NO of having absolute calibration based on the known extinction coefficient for ozone at ultraviolet wavelengths. The invention discloses both static and dynamic flow systems, and the NO concentration measurements may be made over a wide pressure range.

FIELD OF INVENTION

[0001] The present invention relates to gas analysis, and moreparticularly to the detection and measurement of nitric oxide (NO) ingases such as air and human breath by measuring ozone depletion. Asuitable application of the invention is the measurement of theconcentration of NO in the inhaled or exhaled air of a human being orother living organism. Another suitable application is the measurementof NO in ambient air for air pollution monitoring and for scientificstudies of atmospheric chemistry. The invention also applies tomeasurements of nitrogen dioxide (NO₂) in a gas mixture such as air whenNO₂ is first reduced to NO via a photolytic or chemical reaction. Inambient air, the invention may be used to measure the sum of NO and NO₂concentrations, commonly referred to as NO_(x) The invention alsoapplies to the measurement of reactive oxides of nitrogen such as NO₂,nitrate radical (NO₃), dinitrogen pentoxide (N₂O₅), nitrous acid (HNO₂),nitric acid (HNO₃), peroxynitric acid (HNO₄), peroxyacetyl nitrate(PAN), chlorine nitrate (ClNO₃) and particulate nitrate, collectivelyreferred to as NO_(y), either separately or in combination. Thesenitrogen oxide species may be caused to produce NO in a chemicalreaction, as in the reaction at a heated molybdenum oxide surface or inthe reaction at a heated gold surface in the presence of a suitablereducing agent such as hydrogen or carbon monoxide (CO). The presentinvention measures the concentration of NO produced in such reactions.

[0002] The invention may be applied to the detection and quantificationof any substance that may be treated so as to release gaseous NO. Forexample, it is known in the art of chromatography that many compoundscontaining nitrogen can be heated or reacted with other chemicals toproduce NO gas. The NO produced by heating or by reaction with otherchemicals may be detected and quantified using the invention describedhere. Similarly, various substances containing nitrogen such asfertilizers and chemicals used as explosives will slowly decompose torelease NO gas, and that NO gas can be detected and quantified usingthis invention. The detection of fertilizers and explosives may beenhanced by heating the sample to increase the rate of release of NOgas.

[0003] The invention also applies to the detection and quantification ofchemical compounds that do not contain nitrogen themselves but that willreact with a nitrogen-containing compound to produce NO gas. An exampleis the detection and quantification of carbon monoxide (CO) where aircontaining CO is mixed with a nitrogen-containing compound such as NO₂and heated in the presence of a catalytic surface such as a gold surfaceto produce NO. The NO thus produced can be detected and quantified usingthis invention as a means of detecting and quantifying the CO in the airsample.

[0004] More generally, this invention applies to the measurement of theconcentration of any chemical species in a gas such as air if that gasreacts with ozone at a sufficient rate to cause a measurable change inthe ozone concentration. Examples of other chemical species that may bequantified include but are not limited to alkynes and alkenes such asacetylene, ethylene and propylene, compounds commonly found inpetrochemical feedstocks.

BACKGROUND OF THE INVENTION

[0005] Scientific work over the past decade has demonstrated that theconcentration of NO in human breath can be a good indicator ofinflammation in the lungs caused by asthma and other respiratorydiseases. As a result, there is presently a need for a simple,lightweight, low cost instrument for the measurement of NO in humanbreath. This invention addresses that need in particular, but is alsoapplicable to the measurement of NO, NO₂ NO_(x), NO_(y) and variousgases that react with ozone that are present in air and other gasmixtures such as cylinders containing compressed gases and petrochemicalfeedstocks for chemical synthesis.

[0006] At present, the concentration of NO in a gas sample such as airis most commonly measured by mixing the gas sample with air or oxygencontaining ozone gas at low pressures. In a reaction chamber, nitricoxide molecules react with ozone (O₃) molecules, to form nitrogendioxide (NO₂) and oxygen (O₂) molecules. A small fraction of thosereactions also results in the emission of photons having a red ornear-infrared wavelength. The concentration of NO in the gas sample isdetermined by measuring the intensity of that photon emission. Thistechnique, referred to as the “NO+O₃ Chemiluminescence” technique ishighly sensitive and widely used in the measurement of NO concentrationsin ambient air and in inhaled and exhaled human breath. The principaldisadvantages of this technique are: 1) a vacuum pump is required,making the instrument large, heavy and highly consumptive of electricalpower; 2) a cooled, red-sensitive photomultiplier tube is required,adding to the bulk and weight of the instrument and making it relativelyexpensive; and 3) the mixing ratio of ozone required for sensitivedetection is high, typically a few percent, and requires a high-voltage(several hundred volts) electrical discharge to produce the ozone,thereby increasing the risk of human exposure to this toxic gas and tothe danger of electrical shock.

[0007] Another technique for measuring concentrations of NO in airsamples involves contacting a gas sample with an alkaline luminolsolution. As with the ozone-based method described above, this techniqueproduces chemiluminescence. This approach has the advantage of notrequiring a vacuum pump and of detecting photons in the visible regionwhere the photomultiplier tube need not be cooled. However, sensitivedetection using this technique requires the use of chromium (VI) oxideCrO₃ to oxidize NO to NO₂ prior to contact with the luminol solution,and measures must be taken to eliminate large interferences in themeasurement from CO₂and water vapor, both of which are present inexhaled breath at high concentrations.

[0008] This invention makes use of the same chemical reaction used inthe conventional NO+O₃ chemiluminescence instrument commonly used in airpollution monitoring and breath analysis. However, the invention differssignificantly from that instrument in that the basis of detection is notchemiluminescence (detection of photons emitted by the reaction).Instead, the invention measures the decrease in the concentration ofozone that occurs in the chemical reaction. Advantages of this inventionover the conventional NO+O₃ chemiluminescence technique are: 1) theconcentration of ozone required is much lower, in the lowpart-per-million range rather than the percent range; 2) the instrumentcan be operated at any pressure, such as ambient atmospheric pressure,and therefore does not require a vacuum pump; 3) a photomultiplier tubeis not required; and 4) the instrument can be based on the extinction ofUV light and, therefore, is potentially self-calibrating, which mighteliminate the need for compressed cylinders containing standardconcentrations of NO.

SUMMARY OF THE INVENTION

[0009] The main aspect of the present invention is to provide a simpleozone depletion measurement method and apparatus to detect NOconcentration in a test sample.

[0010] Another aspect of the present invention is to use known UVinstruments to detect levels of O₃.

[0011] Another aspect of the present invention is to provide a flexibletest chamber for use as a portable home test kit for lung diseasepatients.

[0012] Another aspect of the present invention is to provide batch andcontinuous flow methods and apparatus to measure O₃ depletion.

[0013] Other aspects of this invention will appear from the followingdescription and appended claims, reference being made to theaccompanying drawings forming a part of this specification wherein likereference characters designate corresponding parts in the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic layout of a theoretical test chamber to mixNO and ozone (O₃) in a static system.

[0015]FIG. 2 is a flow chart of the basic steps of the preferredembodiment method in the static system.

[0016]FIG. 3 is a schematic layout of an alternate embodiment chamberhaving an expandable wall to provide a test chamber that does notrequire a vacuum or a pressurized environment.

[0017]FIG. 4 is a schematic layout of a alternate embodiment chamberusing a UV absorption system to detect concentrations of O₃.

[0018]FIG. 5 is a schematic layout of an alternate embodiment chamberusing a combination of the flexible wall and the UV absorption system.

[0019]FIG. 6 is a schematic layout of an alternate embodiment testapparatus using a continuously flowing gas stream measurement techniquewhich provides batch data analysis.

[0020]FIG. 7 is a flow chart of the basic steps to practice theinvention using the FIG. 6 apparatus.

[0021]FIG. 8 is a schematic layout of the FIG. 6 apparatus using a UVabsorption O₃ measurement technique, the initial prototype.

[0022]FIG. 9 is a schematic layout of an enhanced continuous flowapparatus which constantly measures O₃ concentration in the test gas ina pre- and post-reaction chamber.

[0023]FIG. 10 is a flow chart of the steps to practice the inventionwith the FIG. 9 apparatus.

[0024]FIG. 11 is a sample chart of a theoretical test result using theFIG. 8 apparatus.

[0025]FIG. 12 is a sample chart of a theoretical test result using theFIG. 9 apparatus.

[0026]FIG. 13 is a schematic layout of an alternate embodiment testapparatus using a continuous flowing gas stream measurement techniquewith a variable reaction volume.

[0027]FIG. 14 is a schematic layout of an alternate embodiment testapparatus using a continuously flowing gas stream measurement techniquewith a NO scrubber which provides batch data analysis.

[0028]FIG. 15 is a flow chart of the steps to practice the inventionwith the FIG. 14 apparatus.

[0029]FIG. 16 is a schematic layout of an alternate embodiment testapparatus using a continuous flowing gas stream measurement techniquewith a NO scrubber that provides continuous data analysis.

[0030]FIG. 17 is a flow chart of the steps to practice the inventionwith the FIG. 16 apparatus.

[0031]FIG. 18 is an actual chart of data obtained using apparatus 80 ofFIG. 8.

[0032] Before explaining the disclosed embodiment of the presentinvention in detail, it is to be understood that the invention is notlimited in its application to the details of the particular arrangementshown, since the invention is capable of other embodiments. Also, theterminology used herein is for the purpose of description and not oflimitation.

DETAILED DESCRIPTION OF THE DRAWINGS

[0033] Referring first to FIG. 1 it is understood that the reactionO₃+NO→NO₂+O₂+light energy occurs inside reaction chamber 1. However, thepresent invention does not measure the light energy as does the priorart. Rather, the present invention measures a decrease in O₃concentration which equates to a concentration of NO in the test sample.This O₃+NO reaction occurs within batch system 10 in reaction chamber 1.First ozone (O₃) is added to the test chamber 1 via ozone inlet 3. Thegeneric ozone meter 5 measures the O₃ concentration. Then the patient109 exhales his breath sample via sample inlet 2 into the test chamber1. The test sample contains an unknown amount of NO. The meter 5measures the concentration of O₃ after the O₃+NO reaction issufficiently complete to determine the concentration of NO in the breathsample. The tube 6 feeds the ozone meter 5 the gases from the testchamber 1. The exhaust port 4 is opened after the measurements are done.Known in the art are several valving methods to accomplish the abovemeasurements. For example, the test chamber 1 could be run at a vacuumpressure with the appropriate valves at tubes 2, 3 and 4. The abovesystem is an example of a static rather than a continuous flow system.

[0034] Practice of the present invention requires knowledge of thereaction rate of NO+O₃ and the extinction coefficient for O₃; both ofwhich are well known in the art. Alternatively, calibration standardscould be used to establish the concentration of NO in the sample.

[0035] If the reaction takes place but is not complete, a correction maybe applied according to the formula[NO]_(sample)=[NO]_(measured)/{1−exp(−k[O₃]t)} where NO_(measured) isthe concentration of NO measured, [NO]_(sample) is the actual NOconcentration in the gas sample, [O₃] is the concentration of O₃ and tis the contact time between NO and O₃. k is the second order rateconstant for the gas-phase reaction of NO with ozone. This calculationassumes pseudo-first order kinetics with [O₃]>>[NO]. If this conditiondoes not hold, then one skilled in the art can apply the well-knownresults of second-order kinetics to correct for the concentration of NOthat remains unreacted. Referring next to FIG. 2 the static or batchprocess using the FIG. 1 equipment is described. The first step isnumbered 20. Step 20 calls for adding O₃ to test chamber 1. Step 21calls for using ozone meter 5 to measure the O₃ concentration in thetest chamber 1. Step 22 calls for adding an air sample with an unknownconcentration of NO in it. FIG. 1 shows the measurement of a humanbreath sample. Step 23 calls for allowing a period of time to pass toallow the O₃ to react with the NO. This reaction decreases theconcentration of O₃ in test chamber 1. Step 24 calls for the measurementof the O₃ concentration after the O₃+NO reaction is sufficientlycomplete. Step 25 calls for a comparative calculation of the O₃concentration before and after the test sample was added into the testchamber 1. The calculation yields actual NO concentration as the belownoted example indicates. Step 26 calls for the evacuation of the testchamber 1 to prepare for another measurement. Known in the art areseveral ways to clear the test chamber 1.

[0036] Referring next to FIG. 3 another batch system 30 functions thesame as the FIG. 1 system and the FIG. 2 process. The new feature inFIG. 3 is the flexible membrane 7 on at least one side of the testchamber 1. This flexible membrane 7 could also be built as an entirelyflexible polymer bag. The benefit of the flexible membrane 7 is to allowa user 109 to blow his breath into inlet tube 2 and expand the flexiblemembrane 7, assuming all other tubes 3, 4 are short. This expansioneliminates the need for lab equipment to evacuate the test chamber 1before the test. Thus, the batch test apparatus 30 is suited for aninexpensive home use kit for patients who require daily monitoring oftheir lung condition.

[0037] Referring next to FIG. 4 the batch systems of FIGS. 1, 2, 3 maybe configured to use an ultra-violet (UV) light sensing meter to detectozone concentration. The batch apparatus 40 represents this UV system. AUV light source 8 emits UV light through an optional lens 9 whichtransmits the focused light rays through left window 32 in the testchamber 1. The light travels through the test chamber 1 and is absorbedby the O₃ to decrease the light intensity according to the Beer-Lambertlaw. The light then travels out right window 32 through an optionalfilter 11 and into a photodetector 12. The photodetector 12 may be aphotodiode or a photomultiplier tube known in the art.

[0038] Referring next to FIG. 5 a preferred home use batch apparatus 50comprises a flexible membrane 7 in a test chamber 1 plus a UVmeasurement system as described in FIG. 4.

[0039] Referring next to FIGS. 6, 7 a continuously flowing gas system 60is shown. The designation “conduit 14” represents the common pathwayvolume of the O₃+air sample without the large volume reaction chamber15. The theory of operation is that when a test sample or a continuouslyflowing sample is sent through conduit 14, little or no O₃ reacts withNO, and a baseline O₃ measurement is established. When a second testsample or a continuously flowing sample is sent through the large volumereaction chamber 15, practically all of the NO is reacted with O₃, thusyielding an accurate NO concentration by a comparison of the twomeasurements. Optimally, the volume of conduit 14 would be zero, so that0% reaction of O₃+NO would occur while the gases transit this pathway,and the volume of reaction volume 15 would be infinite, so that thisreaction would proceed to 100% completion. In this simple model, theconcentration of NO would then be the simple difference of the O₃concentrations measured by ozone meter 5.

[0040] An alternative embodiment to conduit 14 is the small volumereaction chamber 61. The actual volume of chamber 61 is chosen such thatthere is sufficient time for potentially interfering compounds to reactto a large extent with O₃ but not enough reaction time for a significantamount of NO to react with O₃. Examples of potential interferences whoseeffects can be eliminated in this way include some alkenes such asethylene.

[0041] In practice, the conduit 14 may actually be a small volumereaction chamber 61 which serves to cancel out the effects ofinterfering compounds by allowing those compounds to react with anapproximately equal amount of ozone in the conduit (or small reactionvolume) and the large volume reaction volume.

[0042] A sample inlet 2 contains an unknown NO concentration perhapsfrom a long human breath exhalation. Ozone is supplied either by aninlet 3 or an in-line O₃ generator 16 which could be a photochemicalreactor. Valve 13 diverts the combined flow from inlets 2 and 3 toeither the conduit 14 (or small volume reaction chamber 61) or the largevolume reaction chamber 15 via connecting tube 6. The ozone meter 5 isconnected to the exhaust port 4.

[0043] Step 70 of FIG. 7 shows a test sample of gas initiated intosample inlet 2. Decision step 71 provides for either a continuous O₃flow through inlet 3 in step 72, or for in situ production of O₃ withthe O₃ generator 16. In step 73 O₃ and the test sample are sharing thetube at tube segment 62, and the valve 13 is first set to flow themixture through the conduit 14 (or small volume reaction chamber 61).Step 74 measures the output from the conduit 14 (or small volumereaction chamber 14) with ozone meter 5.

[0044] Step 75 sets valve 13 to the large volume reaction chamber 15 sothat a second O₃ measurement can be made in step 76. Step 77 comparesthe two measurements to determine the NO concentration in the continuoussample of gas. Repeat measurements can be made by returning to step 73.

[0045] Referring next to FIG. 8 the same procedure described by FIG. 7can be used. The generic ozone meter 5 of FIG. 6 has been replaced witha detection cell 17 connected at tube segment 63. The known means tomeasure O₃ in the detection cell 17 consists of shining a UV light 8through an (optional) lens 9, through the left UV transparent window 32,through the test sample in detection cell 17, out the right UVtransparent window 32, through the (optional) optical filter 11, andinto photodetector 12 (a photodiode, photomultiplier tube or theequivalent). An optional gas pump 18 can be used to draw the gas throughthe apparatus.

[0046] Referring next to FIGS. 9, 10 a continuous flow system 90 usesthe small volume reaction chamber/large volume reaction chamber concept,but sends the same gas sample through a first measurement in referencedetection cell 19 and a second measurement in detection cell 17. ArrowsF1, F2 indicate the gas flow directions.

[0047] Step 100 initiates a continuous gas sample into inlet 2. Decisionstep 101 provides for either a continuous O₃ flow into inlet 3, or insitu O₃ production in O₃ generator 16. The test sample and O₃ mixture atconduit 14 (or small volume reaction chamber 61) are fed into thereference detection cell 19 and the O₃ concentration is measured in step103. The same gas sample travels into the large volume reaction chamber15, and the O₃ and NO react. Step 104 measures the reacted gas mixturein detection cell 17.

[0048] Step 105 does the comparison of the two measurements to determinethe NO concentration in the test sample. Repeat measurements are done byreturning to step 103.

[0049] Referring next to FIG. 11, the theoretical response to twosamples of human breath is shown using the NO detection system 60 ofFIG. 6. The baseline signal is obtained when the solenoid valve 13 ispositioned such that the ozone mixed with sample air passes throughconduit 14 (or the small volume reaction chamber 61). The positiveexcursions in signal (points 111-120) are obtained when the solenoidvalve 13 is positioned such that ozone mixed with sample air passesthrough the large volume reaction chamber. For this NO detection system,the NO measurement in breath is intermittent, where the measurementsreturn to “BASELINE” after each breath sampling period 111-120. However,if the baseline is sufficiently stable, the breath sampling period couldcover the entire time of expiration of an entire human breath.

[0050] Referring next to FIG. 12, the theoretical response to twosamples of human breath is shown using the NO detection system 90 ofFIG. 9. For this NO detection system, the NO measurement is continuousbecause the “BASELINE” is continuously measured in detection cell 17 andits value subtracted from the continuously measured sample value inreference cell 19.

[0051] Referring next to FIG. 13, the NO detection system 130 is analternative to the NO detection system 60 of FIG. 6. In this NOdetection system the solenoid valve, conduit and large volume reactionvolume are replaced by a single variable volume reaction chamber 20(such as bellows or combination of piston and cylinder) that can becompressed to form a conduit (or small volume reaction chamber) andexpanded to form a large volume reaction chamber. Shrinking andexpanding the variable volume reaction chamber 20 of FIG. 13 isequivalent to switching the solenoid valve 13 of FIG. 6. When a bellowsis used, this NO detection system has the advantage that the surfacesexposed to the ozone mixed with gas sample are the same for the conduit(or small volume reaction chamber) and large volume reaction chamber.This could be important because it is known in the art that ozone can bedecomposed on surfaces, and a difference in ozone destruction at theconduit (or small volume reaction chamber) and large reaction volumescould result in a false NO signal.

[0052] Referring next to FIGS. 14, 15 the NO detection system 140 isanother alternative to the NO detection system 60 of FIG. 6. In this NOdetection system a solenoid valve 13 is alternately switched such that agas sample entering inlet 2 (Step 81) passes through either bypass tube600 (Step 87) or NO scrubber 28 (Step 82). The NO scrubber 28 containsmaterials known in the art to remove NO from the gas sample. Followingthe connecting tubing or scrubber, ozone is added to the gas samplethrough inlet 3 (Step 84, 89) or alternatively is produced directly inthe flowing stream using photochemical reactor 16 (Steps 83, 88). Next,the mixture of gas sample and ozone flows through the large volumereaction chamber 15 to allow NO to react with O₃ (Steps 85, 91) and isthen detected using the generic ozone meter 5 (Steps 86, 92). The NOconcentration is calculated as the ozone concentration measured when thegas sample passes through the NO scrubber 28 minus the ozoneconcentration measured when the gas sample passes through bypass tube600 (Step 93). A correction can be applied if the reaction time isinsufficient for NO and O₃ to react completely in the large volumereaction chamber.

[0053] Referring next to FIGS. 16, 17 the NO detection system 148 isanother alternative to the NO detection system 60 of FIG. 6. In this NOdetection system, a continuously flowing gas sample entering the systemthrough sample inlet 2 (Step 141) is split into two continuously flowinggas streams (Step 142). One of the parts of the continuously flowing gasstream flows through a NO scrubber 28 (Step 143). Gas containing ozoneis continuously added to this partial continuously flowing gas samplethrough inlet 3 (Step 145). The partial continuously flowing gassample+ozone then flows through the left large volume reaction chamber15, which allows ozone to react with any interfering species which maybe present in the continuously flowing gas sample (Step 147), and alsocompensates for the time required for the remainder of the continuouslyflowing gas sample+ozone to flow through the apparatus. It is well knownin the art that this latter timing issue can also be addressed throughelectronic manipulation of the measurement data. After passing throughthe large volume reaction chamber 15, the ozone in the continuouslyflowing gas sample+ozone is detected using left generic ozone meter 5(Step 149), and the partial continuously flowing gas sample+ozone exitthe instrument via left outlet 4. Simultaneously to the aforementionedsteps involving the partial continuously flowing gas sample, theremainder continuously flowing gas sample passes through bypass tube6000 (Step 144). Gas containing ozone, split from the source supplyingthe aforementioned partial continuously flowing gas sample iscontinuously added to the remainder continuously flowing gas samplethrough inlet 3 (Step 146). The remainder continuously flowing gassample+ozone then flows through the right large volume reaction chamber,where the ozone reacts with the NO present in the remainder partialcontinuously flowing gas sample (Step 148). The ozone is then detectedusing right generic ozone meter 5 (Step 150), and the remaindercontinuously flowing gas sample+ozone exit the instrument via rightoutlet 4. The NO concentration in the continuously flowing gas sample iscalculated as the difference between this measurement and that obtainedby left generic ozone meter 5 (Step 151). A correction can be applied ifthe reaction time is insufficient for NO and O₃ to react completely inthe right large volume reaction chamber.

EXAMPLE

[0054] Referring next to FIG. 18, experimental results for measurementsof NO are shown for the NO detection system 80 of FIG. 8. In thisexperiment only the large volume reaction chamber 15 was used, and thebaseline ozone was measured by removing the sample NO from inlet 2rather than by passing the ozone and sample air mixture through thesmall reaction volume. Ozone was generated by use of a photochemicalreactor (not shown but located directly on inlet 3) and admitted to thesystem via inlet 3. Measurements of ozone were made every 10 seconds byabsorbance of light having a wavelength of 254 nm. With no sample airadded, the ozone concentration was in the range 3550 to 3650parts-per-billion by volume (ppbv); see “BASELINE O₃ CONCENTRATION.” Thetotal flow rate of sample air and O₃ was 0.95 L/min, the volume of thelarge volume reaction chamber was 100 cm³, and the total pressure was0.71 atm, resulting in a reaction time of 4.5 seconds. Using thewell-known rate constant for the NO+O₃ reaction of 1.8×10⁻¹⁴ cm³ molec⁻¹s⁻¹, it is calculated that 99.4% of the NO in the sample reacts with O₃.Air was sampled three times each for five different NO concentrationslabeled groups I, II, III, IV, V. Ozone concentrations declined eachtime sample air was admitted to the system, corresponding to measured NOconcentration group I=1317, group II=651; group III=309, group IV=97;group V=31 ppbv for the five different air samples tested.

[0055] The formula to obtain the NO concentration of 1317 in group 1 isto subtract the average of readings 94, 95, 96 from the average of thebaseline at points 97, 98, 99. The measurement is then corrected bymultiplying by 1.006 to correct for the 0.6% of NO that does not reactwithin the large volume reaction chamber.

[0056] Although the present invention has been described with referenceto preferred embodiments, numerous modifications and variations can bemade and still the result will come within the scope of the invention.No limitation with respect to the specific embodiments disclosed hereinis intended or should be inferred. Each apparatus embodiment describedherein has numerous equivalents.

We claim:
 1. A method to determine a NO concentration in a test sample,the method comprising the steps of: measuring a concentration of O₃ in agas sample; adding a test sample to the gas sample; and measuring adecrease of O₃ in a reacted mixture of the gas sample and the testsample to determine the NO concentration in the test sample.
 2. Themethod of claim 1, wherein the gas sample is air and the O₃ is added tothe air before the measuring step.
 3. The method of claim 2, wherein thetest sample is a human breath.
 4. The method of claim 3 furthercomprising the step of using a reaction chamber for the adding of thetest sample to the gas sample.
 5. The method of claim 4 furthercomprising the step of using a reaction chamber having a flexible wall.6. A method to measure a concentration of a chosen gas in a sample ofgas, the method comprising: adding O₃ to a reaction chamber; measuringan O₃ concentration in the reaction chamber; selecting a chosen gas thatreacts with ozone; adding a gas sample containing the chosen gas to thereaction chamber; waiting a period of time for the O₃ to react with thechosen gas; measuring the O₃ concentration in the reaction chamber; andcomparing the O₃ concentration before the addition of the gas sample tothe O₃ concentration after the addition of the gas sample to determinethe concentration of the chosen gas in the gas sample.
 7. The method ofclaim 1 further comprising the step of using a reaction chamber.
 8. Themethod of claim 7 further comprising the step of using a UV ozonedetector for measuring the concentrations of O₃.
 9. The method of claim5 further comprising the step of using a UV ozone detector for measuringthe concentrations of O₃.
 10. (FIG. 6) A method to determine a NOconcentration in a continuously flowing sample of gas, the methodcomprising the steps of: introducing the continuously flowing sample ofgas containing NO into a test apparatus having a conduit and a largevolume reaction chamber; adding O₃ to the continuously flowing sample ofgas; sending the O₃+the continuously flowing sample of gas mixture tothe conduit; measuring an O₃ concentration in or downstream of theconduit; sending the O₃+continuously flowing sample of gas mixture tothe large volume reaction chamber; measuring an O₃ concentration in ordownstream of the large volume reaction chamber; and comparing the O₃measurements of the conduit and large volume reaction chamber todetermine the NO concentration in the gas sample.
 11. The method ofclaim 10, wherein the conduit further comprises a small volume reactionchamber.
 12. The method of claim 10 further comprising the step of usinga UV ozone detector for the O₃ measuring steps.
 13. A method todetermine a NO concentration in a continuously flowing sample of gas,the method comprising the steps of: introducing the continuously flowingsample of gas+O₃ mixture into a reference detection cell; measuring anO₃ concentration in the reference detection cell; sending thecontinuously flowing sample of gas+the O₃ mixture into the large volumereaction chamber; sending the continuously flowing sample of gas+O₃mixture from the large volume reaction chamber into a detection cell;measuring an O₃ concentration in the detection cell; and comparing theO₃ measurements in the reference and detection cells to determine the NOconcentration in the continuously flowing sample of gas.
 14. The methodof claim 13 further comprising the step of continuously flowing the gassample into a small volume reaction chamber prior to sending the gassample+O₃ mixture into a reference detection cell.
 15. The method ofclaim 13 further comprising the step of using a UV ozone detector forthe O₃ measurements.
 16. A method to measure a concentration of a chosensubstance in an original sample, the method comprising the steps of:treating the original sample to form NO in a gaseous state therebyforming a gas sample; measuring a concentration of O₃ in a reactionchamber; adding the gas sample to the reaction chamber; and measuring adecrease of O₃ in the reaction chamber, thereby determining the NOconcentration in the gas sample and further determining theconcentration of the chosen substance in the original sample.
 17. Amethod to detect a nitrogen based explosive in an original sample, themethod comprising the steps of: treating the original sample to form NOin a gaseous state thereby forming a gas sample; measuring aconcentration of O₃ in a reaction chamber; adding the gas sample to thereaction chamber; and measuring a decrease of O₃ in the reactionchamber, thereby determining the NO concentration in the gas sample andfurther determining whether a nitrogen based explosive exists in theoriginal sample.
 18. A method to measure a concentration of a chosennitrogen containing substance in an original sample, the methodcomprising the steps of: treating the original sample to form NO in agaseous state thereby forming a gas sample; measuring a concentration ofO₃ in a reaction chamber; adding the gas sample to the reaction chamber;and measuring a decrease of O₃ in the reaction chamber, therebydetermining an NO concentration in the gas sample and furtherdetermining the concentration of the nitrogen containing chosensubstance in the original sample.
 19. A method to determine aconcentration of a chosen chemical compound in an original sample, saidchemical compound does not contain nitrogen, said chemical compound willchemically react with another compound which does contain nitrogen, themethod comprising the steps of: treating the original sample with anitrogen containing compound to induce a chemical reaction, therebyproducing a gas sample containing NO; measuring a concentration of O₃ inthe reaction chamber; and measuring a decrease of O₃ in the reactionchamber, thereby determining the NO concentration in the gas sample, andfurther determining the concentration of the chosen chemical compound inthe original sample.
 20. (FIG. 13) A method to determine a NOconcentration in a continuously flowing sample of gas, the methodcomprising the steps of: introducing a continuously flowing sample ofgas containing NO into a test apparatus having a variable volumereaction chamber (VVRC) with the VVRC set at a small volume; adding O₃to the continuously flowing sample of gas; sending the O₃+continuouslyflowing sample of gas mixture to the VVRC with the VVRC set at the smallvolume; measuring an O₃ concentration in or downstream of the VVRC withthe VVRC set at the small volume; setting the VVRC to a large volume;sending the O₃+continuously flowing sample of gas mixture to the VVRCwhile it is set to the large volume; measuring an O₃ concentration in ordownstream of the VVRC set at the large volume; and comparing the O₃measurements of the VVRC set at the small volume to the measurements ofthe VVRC set at the large volume to the determine NO concentration inthe gas sample.
 21. The method of claim 20, wherein the VVRC furthercomprises a bellows.
 22. (FIG. 14) A method to determine a NOconcentration in a continuously flowing sample of gas, the methodcomprising the steps of: assembling a test apparatus having a NOscrubber connected to a large volume reaction chamber and having abypass connected to the large volume reaction chamber; sending thecontinuously flowing sample of gas to the NO scrubber; adding O₃ to thecontinuously flowing sample of gas; sending the O₃+continuously flowingsample of gas mixture to the large volume reaction chamber; measuringthe O₃ concentration in or downstream of the large volume reactionchamber; sending the continuously flowing sample of gas to the bypasstube; adding O₃ to the continuously flowing sample of gas; sending theO₃+continuously flowing sample of gas mixture to the large volumereaction chamber; measuring the O₃ concentration in or downstream of thelarge volume reaction chamber; and comparing the O₃ measurements of theNO scrubber measurement and the bypass measurement to determine the NOconcentration in the continuously flowing gas sample.
 23. The method ofclaim 22, wherein the step of adding the O₃ to the gas sample is doneafter the step of sending the continuously flowing gas sample to eitherthe bypass tube or NO scrubber.
 24. The method of claim 23 furthercomprising the step of using a UV ozone detector for the O₃ measuringsteps.
 25. An apparatus to determine a NO concentration in a testsample, the apparatus comprising: means for measuring a concentration ofO₃ in a gas sample functioning to determine an O₃ baseline; means foradding a test sample to the gas sample functioning to create a chemicalreaction between the O₃ and NO; and means for measuring a decrease of O₃in a reacted mixture of the gas sample and the test sample functioningto determine the NO concentration in the test sample.
 26. An apparatusto measure a concentration of a chosen gas in a sample of gas, theapparatus comprising: means for measuring an O₃ concentration in areaction chamber functioning to determine an O₃ baseline; means foradding a gas sample containing a chosen gas that reacts with ozone, tothe reaction chamber functioning to create a chemical reaction betweenO₃ and the chosen gas; and means for measuring a decreased O₃concentration in the reaction chamber after the chemical reaction takesplace functioning to determine the concentration of the chosen gas inthe gas sample.
 27. An apparatus to determine a NO concentration in acontinuous test sample, the apparatus comprising: means for continuouslyflowing a gas sample containing NO into a test apparatus having aconduit and a large volume reaction chamber functioning to fill theconduit and the large volume reaction chamber with the gas sample; meansfor sending O₃+gas sample mixture to the conduit functioning to createan O₃ baseline; means for sending the O₃+gas sample mixture to the largevolume reaction chamber functioning to create a chemical reactionbetween the O₃ and the NO; means for measuring a decreased O₃concentration in or downstream of the large volume reaction chamberfunctioning to determine the NO concentration in the gas sample.
 28. Anapparatus to determine a NO concentration in a continuously flowingsample of gas, the apparatus comprising: means for continuously flowinga gas sample containing NO and O₃ into a conduit functioning to fill theconduit with the gas sampled and O₃; means for sending the continuouslyflowing gas sample+O₃ mixture into a reference detection cellfunctioning to fill the reference detection cell with the gas sample andO₃ mixture; means for measuring an O₃ concentration in the referencedetection cell functioning to create an O₃ baseline; means forcontinuously flowing the gas sample containing NO and O₃ into a largevolume reaction chamber functioning to create a chemical reaction of thegas sample+O₃; means for sending the continuously flowing gas sample+O₃reaction mixture into the detection cell functioning to prepare for anO₃ measurement; means for measuring a decreased O₃ concentration in thedetection cell functioning to determine the NO concentration in the gassample.
 29. (FIG. 16) A method to determine a NO concentration in acontinuously flowing sample of gas, the method comprising the steps of:assembling a test apparatus having a NO scrubber connected to a firstlarge volume reaction chamber and having a bypass connected to a secondlarge volume reaction chamber; simultaneously sending the continuouslyflowing sample of gas to the NO scrubber and first large volume reactionchamber and to the bypass and the second large volume reaction chamber;simultaneously adding O₃ to the first and to the second large volumereaction chamber; measuring an O₃ concentration in the first largevolume reaction chamber; measuring an O₃ concentration in the secondlarge volume reaction chamber; and comparing the O₃ measurements fromthe first and second large volume reaction chambers to determine the NOconcentration of the continuously flowing sample of gas.